A primary concern in the geologic storage of anthropogenic carbon dioxide is the leakage of buoyant CO₂ plumes into shallower formations, aquifers, or the surface. Man-made wells drilled through these formations present a potential leakage pathway for this CO₂ as the cement binding the well to the earth develops fractures or debonded microannuli form over time. Typically, wells with poor cementing or suspected leaks are subject to a cement-squeeze, in which new cement is injected to eliminate the leakage pathway. However, small fractures or leakage pathways are often difficult for oilfield cement to repair, as the cement dispersion is potentially screened out from dispersing fluid and cannot enter the fracture. Therefore a low-viscosity sealant is desired that can enter these leakage pathways easily and provide a robust seal. A class of poly(acrylic acid) polymers known commercially as Carbopol® are pH-sensitive microgels and swell/thicken upon neutralization with alkali cement components. These polymer dispersions are tested for ease of placement into cement fractures and subsequent development of resistance to displacement. Laboratory experiments involved injecting various unswollen polymer microgel dispersion into constructed cement fractures while measuring injection pressure and the pH of the polymer effluent to quantify the chemical reactions taking place and the induced viscosity changes. Fractures were constructed in order to allow for visual inspection of the polymer microgel swelling during and after injection, qualitatively useful in determining the polymer’s efficiency at blocking cement fractures. It was determined that polymer microgels undergo syneresis in the presence of calcium cations that are dissolved from minerals present in cement. The syneresis causes the polymer to collapse onto the cement fracture face and expelled water is left to fill the rest of the fracture, providing little to no resistance to subsequent flow. However, the syneresed polymer does show some potential in blocking or partially blocking small aperture fractures and is not entirely detrimental to fracture blockage in small amounts. An acid pre-flush prior to polymer injection has been seen to favorably reduce the amount of calcium and therefore extent of syneresis, allowing swollen polymer microgels to remain intact and block fluid flow. / text

Oil companies need to evaluate the risk of annular fluid or gas migration if cement fails during the life of the well. Sustained casing pressure can lead to shutting in the wells to avoid health, safety, and environment (HSE) risks and government fines. To understand the long-term integrity of cement in high temperature and high pressure (HTHP) conditions and the mechanical properties that affect the ability of cements to seal fluids, this project used finite-element models (FEMs) to study the stress-causing phenomena. FEM analyses in ABAQUS version 6.11 were used to determine the potential of cement failure in oil wells. The model uses a 3D section of a well that can be used for different casing and formation types under different loading conditions. The model built in ABAQUS version 6.11 allows incorporating materials with nonlinear mechanical properties; it also uses FEM analysis to forecast fractures inside the cement under different loading scenarios like hydraulic fracture jobs or casing tests. The finite-element model included cases for cement cracking, cement debonding, and plastic deformation of the cement and rock that can generate loss of zonal isolation. Linear manner: set cements behave elastically until a failure criterion is reached, and then they can behave plastically. The FEM approach can reproduce stresses, strains, and volume changes in the material under different environmental HTHP conditions. Cemented wells have both tensile and compressive stresses that make some parts of the cement sheath experience fracture initiation, plastic deformation, or debonding. This dissertation provides a model that will help drillers design the set cement for long-term integrity in HPHT well conditions. The FEM predicts if the cement sheath can develop debonding, cracks or plastic deformations during the life of the well. The cement sheath needs to be designed for long-term zonal isolation to avoid interzonal communications, remedial costs and environmental problems related to cement seal. A CMS™-300 Automated Permeameter, a mechanical properties analyzer, HPHT cement consistometer, annular expansion molds, and tri-axial test equipment were used in this study to test cements for specific applications in three Colombian oil fields, including an oil field with in-situ combustion project.